The required column types are x and y (ie the default given by Hypatia).
Each of the values for this attribute may be either a string (indicating one column) or an array reference of strings (indicating several columns).
In the latter case,
the number of x and y columns must match and each respective x and y column will form its own bar chart.
In the former case,
the single x column will act as the same x column for all of the y columns.